USRPDevice: Fix setRxGain return on error and getRxGain() returning always 0

field rxGain is set to 0 during constructor and never set after that
point.

Change-Id: I7fae7a315e5ab98a15c27628a88a92226ef89469

Transceiver52M/device/usrp1/USRPDevice.cpp

@@ -305,10 +305,11 @@ double USRPDevice::setRxGain(double dB, size_t chan)
 
   if (!m_dbRx->set_gain(dB))
     LOGC(DDEV, ERR) << "Error setting RX gain";
-
+  else
+    rxGain = dB;
   writeLock.unlock();
 
-  return dB;
+  return rxGain;
 }
 
 bool USRPDevice::setRxAntenna(const std::string &ant, size_t chan)